You arrive on a hot day at a foreign airport. After you wheel your bag past a bored customs official, glass doors slide open to reveal the line of sweaty cab drivers who hold up notices carrying strange names. What if, instead of making a frantic search for the saviour who brandishes yours, you chose somebody's else's sign, somebody else's driver - somebody else's life? Good books and plays tend to begin with strong and clear ideas. As does Michael Frayn's 11th novel, Skios.

"Over the years," he says, "every time I've arrived in an airport somewhere, and seen that line of people holding up placards with names, I've thought: 'supposing I went up to one of them and said I was somebody I wasn't'. They obviously don't know what the people they're waiting for look like."

Oliver Fox, Frayn's floppy-haired wastrel with a string of enraged girlfriends and a goofily seductive style, does just that when he snatches the identity of an eminent scientist due to speak at a high-minded symposium on a small Greek island. Cue a classic Fraynian farce, as richly laden with ideas as with plot-twists that hairpin and zigzag like the dusty roads of Skios itself.

The book itself began as an unexpected detour. Frayn and his wife, the biographer Claire Tomalin, went on holiday to a Greek island so he could work on his droll, touching and bittersweet family memoir, My Father's Fortune.

Then, "in the maddening way that one's mind works, I suddenly began to think about this story, and it began to fall into place". My Father's Fortune had to wait while Frayn mapped out the destiny of the impostor Oliver and his hapless victim: the guru of "scientometrics", Dr Norman Wilfred. The Fred Toppler Foundation, an ersatz temple to "European civilisation", in reality serves as the island laundry for the grubbier deals of a Greek plutocrat. It teeters and topples as Oliver brings his own scatty version of chaos theory to bear on "the great gear-chain of cause and effect".

Frayn notes that the swashbuckling heroes of adventure yarns often serve as "wish-fulfilments, and have the kind of superhuman courage that their authors don't have". Well, Oliver never lacks - if not courage, then sheer chutzpah: "The hero of this book, although he's not a terribly morally admirable man, does have the cheek to do what one sometimes wouldn't mind doing oneself." Indeed, his tightrope act of imposture trumps the usual fraudster's tricks.

As always in Frayn's philosophical comedies, the alarms and excursions of a breakneck intrigue drive everyday reality to its logical - and illogical - conclusions. What scoundrelly Oliver does, reflects Frayn, "is really an exaggeration of what we're all doing all the time. We're all playing some kind of part, and we don't really know what kind of part it is we're supposed to be playing. We're trying to find it out, and to feel from other people's reactions whether we're doing the right thing - trying to find some kind of plausible way to be in life."

Does being Michael Frayn, I ask, ever feel like an onerous performance? Would he like to appoint an understudy? "You do get used to it," he muses. "Practice makes perfect."

Frayn, who will be 80 next year, enjoys more practice than ever at acting his many parts. Skios is his first novel for a decade, since Spies re-cast the author's wartime childhood into another trapdoor-strewn story of misplaced conviction and mistaken identity. After that, The Human Touch summed up, over 500 nourishing but joke-studded pages, his philosophy of events, of happenings, of process: the narratives that allow a story-spinning species to make sense - and make art - of its world.

As Skios returns to some favourite Frayn terrains - where chance runs into choice, determinism wrangles with free will, and the random blows of fate shake our habitual need to construct plots - I wonder how far this supremely cunning storyteller also has a message to impart. Unlike the clunkily didactic Dr Wilfred, his creator has no fondness for delivering lectures: "I'd like to think I'd got some sort of programme for teaching people philosophy - but I'm afraid I haven't. Ideas just come to one's head and one tells a story the best one can."

My Father's Fortune crafted a literary persona that many readers found warmer, and more vulnerable, than the cerebral clock-maker of some caricatural Frayn portraits. It placed not only his father's life as a jaunty self-creator at its heart, but also his mother's sudden death. The memoir proved "a much more emotional experience" than some other works - "but it also made me laugh a lot, thinking about my father and our efforts to get on together".

Always a wizard of dialogue, Tom Frayn's son lets us listen to this insecure, eccentric, class-shifting family: "I could hear his voice very clearly, and I could hear my stepmother's voice, and the voice of everyone except my mother ... I did find that very painful. I couldn't begin to write dialogue for her."

Meanwhile, his career as a playwright has entered an Indian summer without end. The latest West End version of the evergreen Noises Off, at the Novello in London until the end of June, delivers nightly delight. At the Rose in Kingston, the revival of his (rewritten) Here elicits thrilled praise from the author for director Lisa Spirling and a cast that includes Alison Steadman.

In the theatre, as in his fiction, Frayn attracts bouquets - and the odd brickbat - for his steely control of the machinery of art. Yet his customary first-night nerves serve as a reminder that he has always chosen to work in the form that leaves both authors and actors most exposed: comedy. The mission to amuse leaves the laugh-seeker naked. Comedy has no court of appeal, no final arbiter. People find it funny, or they don't.

That is "Both the curse and the consolation of comedy," thinks Frayn. "If people don't laugh, that's extremely painful. But if they do laugh, then that's a justification in itself. If people are sitting there quietly, are they moved internally? You never know. But with comedy, you do. By God, you do.

"All plots," he insists, "including farce plots, depend upon perception. People are not moved as by pieces of machinery. They see something happening and they understand it in a certain way. It does involve human understanding, and human invention too." This master of mechanics, in fact, writes as a champion of happenstance.

We can, and must, deviate from our paths of plot - even at talking-shops such as the shindig on Skios. "The only time I ever went to a writers' conference," Frayn recalls, "was in Sweden, years and years ago. I thought it was such a ridiculous experience that I've never repeated it." A flock of distinguished authors ran out of things to say on topics such as "Language as a Home". "We got into such desperate straits that there were long silences." Finally, the German poet and critic Hans Magnus Enzensberger exercised his - and their - existential freedom. "He stood up and said, 'I suggest we end the formal part of the conference and all go out and walk about in the sunshine'." So they did.